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Physics of the Earth and Planetary Interiors 158 (2006) 190–209
Lithospheric and sublithospheric anisotropy beneath
central-southeastern Brazil constrained by
long period magnetotelluric data
Antonio L. Padilha ∗ , Ícaro Vitorello, Marcelo B. Pádua, Maurı́cio S. Bologna
Instituto Nacional de Pesquisas Espaciais, INPE, CP 515, 12201-970 São José dos Campos, Brazil
Received 12 July 2005; received in revised form 12 December 2005; accepted 8 May 2006
Abstract
Electric anisotropy calculated from geoelectric strikes of magnetotelluric (MT) data and seismic anisotropy derived from shearwave splitting parameters are jointly analyzed to estimate the degree and orientation of strain in the subcontinental mantle of
central-southeastern Brazil. High-quality long-period MT soundings are available at 90 sites concentrated along four profiles at
the southwestern and southern borders of the Paleoproterozoic-Archean São Francisco craton. This data set is complemented
by a previous study of SKS and SKKS splitting measurements available at more than 40 sites covering a slightly wider region.
For this study, the MT data were processed with modern techniques, including recovery of the undistorted EM field polarizations
through correction of static shift and phase mixing due to galvanic distortions. Three-dimensional forward modelling of MT and GDS
(geomagnetic depth soundings) transfer functions supports interpretation of deep electrical anisotropy in the region. Magnetotelluric
phase responses of orthogonal propagation modes present slight splitting at long periods for most of the sites, indicative of overall
electrically low mantle anisotropy to depths greater than 250 km. The electrical strike azimuths are parallel to the fast NW directions
of shear-wave splitting along the southern borders of the São Francisco craton, suggesting that the seismic anisotropy also resides
within the same depth range. Since this direction is very different from that of present-day South American westward absolute
plate motion, it is inferred that no significant lateral mantle flow or deformation related to the plate motion is observed under the
study area, either because it is absent or the rigid lithosphere is much thicker than expected. To explain the prevailing electric and
seismic common direction it is suggested that a mechanical coupling between lithospheric and sublithospheric mantle exists. A
relic strain, possibly resulting from ancient continental collision processes, is interpreted to have induced a general alignment of
olivine down to mantle depths beneath the continental rigid plate. The observed slightly enhanced conductive texture could be
associated with hydrogen diffusion along the aligned olivine a-axis, especially within mantle patches subjected to metasomatic
processes. The coincidence of mantle strikes with the trend of surface deformation pattern also suggests that crust, lithospheric
and sublithospheric upper-mantle have deformed and translated coherently, preserving the regional NW direction since the tectonothermal event responsible for the deformation. Distinct electric azimuths from the general NW trend are observed in regions probably
perturbed by Cretaceous rift-related intraplate magmatism and in zones of complex deep structures produced by superposed nearly
simultaneous oblique Neoproterozoic collisions in the southeast of the São Francisco craton.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Upper mantle; Central-southeastern Brazil; Magnetotellurics; Electrical anisotropy; Lithosphere/asthenosphere coupling
∗
Corresponding author. Tel.: +55 12 3945 6791; fax: +55 12 3945 6810.
E-mail address: [email protected] (A.L. Padilha).
0031-9201/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.pepi.2006.05.006
A.L. Padilha et al. / Physics of the Earth and Planetary Interiors 158 (2006) 190–209
1. Introduction
Observations of seismic and electrical anisotropy
provide complementary approaches to estimate mantle
deformation and ultimately address a wide range of geological and geophysical problems. Shear-wave splitting
analysis gives information on the azimuthal anisotropy
of the upper mantle, which is generally interpreted as a
consequence of the strain-induced crystallographic texture or lattice preferred orientation (LPO) of intrinsically
anisotropic mantle minerals, principally olivine (Silver,
1996; Savage, 1999). However, the exact location of the
anisotropy within the upper mantle is poorly constrained,
with some researchers interpreting it to reside primarily
within the lithosphere and associated with ancient geological processes (Silver and Chan, 1988; Gaherty and
Jordan, 1995), whereas others suggest it is maintained
by shear deformation in the asthenosphere as a result
of mantle convection (Vinnik et al., 1992; Debayle and
Kennett, 2000).
Similarly, electrical anisotropy may be generated in
the upper mantle either by a preferred interconnection
of a highly conducting mineral phase (such as graphite)
within foliation planes that mark past tectonic events
(Mareschal et al., 1995), or by strain-induced hydrogen
diffusion along olivine crystals oriented by present-day
plate motion (Simpson, 2001). As the depth to an electrically conductive anisotropic-layer is well constrained,
long-period magnetotelluric (MT) soundings can be used
to resolve the ambiguity in interpretation of seismic
anisotropy measurements. However, a careful data analysis must be performed because in some circumstances
lithospheric anisotropy estimated from long period geoelectric strikes can alternatively be explained by crustal
heterogeneity (e.g., Korja, 2003; Heinson and White,
2005).
To date, few MT studies with sounding periods sufficiently long to resolve mantle anisotropy have been
performed in areas with available teleseismic observations. In the Superior Province of Canada, Mareschal et
al. (1995) identified a very significant difference in MT
response in the two orthogonal directions without the
presence of any vertical field response. SKS data from
one region across the Grenville Front indicated that the
fast seismic direction was almost parallel to the high
conductivity direction, yet there was a small but statistically meaningful obliquity between them (Sénéchal et
al., 1996). This obliquity was interpreted by Ji et al.
(1996) as an indicator of the movement sense on ductile mantle shear zones. However, a recent collocated
teleseismic and MT study across the Great Slave shear
zone, northern Canada, did not provide evidence for
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a systematic obliquity between seismic and electrical
anisotropy in the upper mantle (Eaton et al., 2004). For
the North Central craton of Australia, Simpson (2001)
demonstrated the presence of an electrical anisotropy
deeper than 150 km. The direction of electromagnetic
strikes revealed good agreement with the fast directions
of SV-waves (Debayle and Kennett, 2000), but both geophysical datasets showed significant angular discrepancy
with the present-day absolute plate motion (APM).
In central-southeastern Brazil, portable broadband
stations derived upper mantle seismic anisotropy from
measurements of SKS and SKKS splitting at more than
40 sites. The fast polarization directions show consistent
orientation over hundreds of kilometres and are generally parallel to the structural trend of the last major
orogeny (James and Assumpção, 1996; Heintz et al.,
2003). A long-term MT programme is under way in the
same region, aiming at a large-scale reconnaissance of
major conducting geostructures that could have persisted
as records of past episodes (Pádua, 2004; Bologna et al.,
2005, 2006). Ninety high-quality MT soundings have
been deployed along four profiles mainly located in the
southwestern and southern borders of the São Francisco
craton. At most of the stations, the data period ranges
from 0.0008 to 13,653 s, which allows the vertical imaging of geoelectric structures from the near surface (tens
of metres) to great depths into the upper mantle (more
than 250 km).
In this paper, these MT data are analyzed using
the most current techniques to get band-limited strike
directions for periods most sensitive to different
lithospheric/asthenospheric depths. Because the MT
responses at long periods are sensitive both to deep
and to distant structure, dimensionality analysis, tensor
decomposition and three-dimensional (3D) modelling
are carried out in order to understand the effects of crustal
heterogeneity and electrically anisotropic structures in
the crust and in the upper mantle. Selected geoelectric
strikes at typical depths of the crust, upper lithospheric
mantle and deeper lithosphere/asthenosphere are then
compared with seismic anisotropy for interpretations of
the structural deformation below the study area.
2. Geological context
The South American platform is composed of a Precambrian central core, bordered by active subductionrelated orogens to the west and northwest, and of a
Mesozoic-Cenozoic passive continental margin to the
northeast and east. The core is formed by several Archean
to Mesoproterozoic blocks amalgamated during the Neoproterozoic Brasiliano (Pan African) orogeny in the final
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Fig. 1. Generalized geological map of the study area (modified from Schobbenhaus et al., 1984). PB stands for the Paraná basin, TOP for the
Tocantins province, SFP for the São Francisco province, and MQP for the Mantiqueira province. Black dots are the location of the MT sites and
different grey tones represent: 1, Archean; 2, Paleoproterozoic; 3, Mesoproterozoic; 4, Neoproterozoic; 5, Phanerozoic; 6, water.
assembly of Gondwana (Almeida et al., 1981; Alkmim
et al., 2001). In central-southeastern Brazil (Fig. 1), the
São Francisco craton constituted the centre of the West
Gondwana and its margins were initially formed during
the Archean in several discrete events between 3.2 and
2.7 Ga. The Transamazonian orogeny left its imprint on
the southern and eastern parts of the craton between 2.1
and 1.9 Ga, creating a NNW-verging fold-and-thrust belt
that was later incorporated in a dome-and-keel province
(Alkmim and Marshak, 1998).
Following Machado et al. (1996), four principal stratigraphic units occur around the study area
within the São Francisco craton: the Archean basement (granitoid intrusives, gneisses, and migmatites
that range in age from 3.2 to 2.7 Ga), an Archean
(ca. 2.7 Ga) supracrustal assemblage (greenstone and
associated sedimentary sequences), and two Paleoproterozoic supracrustal assemblages (platformal and deep
marine strata including quartzite, phyllite, carbonate, and
Lake Superior type banded iron formations, deposited
between 2.1 and 2.4 Ga, and more recent quartzite and
conglomerate). The basement occurs in dome-shaped
bodies separated by deep keels (troughs) containing
deformed supracrustal rocks. All margins of the craton were subjected to Brasiliano deformation during the
Neoproterozoic and earliest Phanerozoic (Cambrian),
creating fold-thrust belts that generally verge towards
the interior of the craton.
The Tocantins Province is a large Neoproterozoic
Brasiliano orogen developed between three major continental blocks represented by the Amazon and São
Francisco/Congo cratons and another continental block,
presently covered by the Phanerozoic sedimentary and
volcanic rocks of the Paraná basin. The province comprises three large fold belts: the Araguaia and Paraguay
belts, which border the eastern and southeastern margin of the Amazon craton, respectively, and the Brası́lia
belt, established along the western margin of the São
Francisco craton.
Proterozoic metamorphic rock units of varied nature
and age constitute most of the Brası́lia belt, comprising
passive margin sequences of the São Francisco continent,
back-arc and fore-arc basin sequences, and a younger
post-inversion platform sequence deposited in a foreland basin of the São Francisco craton (Pimentel et al.,
2001). In the southern segment of the belt, the Brasiliano
deformation involved overthrust sheets (nappes), transported eastward at least 150 km towards the platform
of the São Francisco craton, and subsequently stacked
by thrust faults over Neoproterozoic pelitic and carbonatic sequence (Fuck et al., 1994). In the study area, the
Brası́lia belt forms a complex structural pattern, locally
characterized by foliations associated with the thrust
sheets and long lineaments corresponding to sub-vertical
lateral ramps or wrench faults originated from differential displacement of the thrust wedges.
A.L. Padilha et al. / Physics of the Earth and Planetary Interiors 158 (2006) 190–209
Extensive Cretaceous magmatism is observed in
the southern portion of the Brası́lia belt and northern
Paraná basin, possibly related to several other Cretaceous
alkaline-carbonatite provinces that evolved contemporaneously around the Paraná basin during the opening of
the South Atlantic Ocean and subsequent westward drift
of the continent (Thompson et al., 1998).
The vast region located between the São Francisco
craton and the eastern continental margin of Brazil is
encompassed by the northern and central sectors of
the Neoproterozoic Mantiqueira Province (Almeida and
Hasui, 1984). This N–NE trending structure has been
viewed as part of the Araçuaı́-West Congo orogen, developed between the São Francisco and Congo cratons in
the course of the Brasiliano assembly of West Gondwana during the Neoproterozoic (Trompette, 1997). It
can be divided into the largely greenschist and amphibolite facies Araçuaı́ belt on the west and the largely
granulite facies Ribeira belt on the east. A pronounced
linear gravity and magnetic anomaly defines the boundary between these two belts.
The major tectonic framework of the Ribeira Belt is
defined by two distinct terrains (Heilbron et al., 2000).
The occidental terrain comprises a pile of superposed
allochtonous terrains thrust to the west and subsequently
deformed in transpressional regime, with large vertical
oblique shear zones associated with granitic plutons. It
is considered as the early São Francisco craton passive
margin. The oriental terrain is characterized by large isoclinal recumbent folds, low angle dipping metamorphic
foliation, and numerous NW trending ductile-ruptile
shear zones containing post-collisional granitoids. To the
west, the Ribeira belt deformation partially overprints the
slightly older Brası́lia belt. The main collisional event of
the Brası́lia belt occurred around 625 Ma, whereas in
the Ribeira belt the main collision took place around
590 Ma (Schmitt et al., 2004). This diachronous evolution is registered by an interference zone between these
belts, to the south of the São Francisco craton, where lowto medium-pressure metamorphic mineral assemblages,
related to the Ribeira belt, overprint higher pressure
metamorphic mineral assemblages of the Brası́lia belt
(Trouw et al., 2000).
To the west, the Paraná basin is a large intracratonic basin, developed entirely on continental crust and
filled with sedimentary and volcanic rocks ranging in
age from Silurian to Cretaceous. Five major depositional
sequences (Silurian, Devonian, Permo-Carboniferous,
Triassic, and Juro-Cretaceous) constitute the stratigraphic framework of the basin. The first four are predominantly siliciclastic in nature, and the fifth contains
the most voluminous basaltic lava flows of the planet.
193
The depositional history of the basin was closed in the
Upper Cretaceous by a package of alluvial, fluvial and
eolian sedimentary rocks. Maximum thickness exceeds
7000 m in the central depocentre. The sequences are
separated by basin wide unconformities related in the
Paleozoic to Andean orogenic events and in the Mesozoic to the continental breakup and sea floor spreading between South America and Africa. The structural
framework of the Paraná basin consists of a remarkable pattern of linear features (faults, fault zones, arches)
clustered into three major groups (N45–65W, N50–70E,
E–W). The northwest- and northeast-trending faults are
long-lived tectonic elements inherited from the Precambrian basement whose recurrent activity throughout the
Phanerozoic strongly influenced sedimentation, facies
distribution, and development of structures in the basin
(Zalán et al., 1990).
3. Magnetotelluric data acquisition and
processing
Over the last 7 years, the natural source MT method
has been extensively used in the central-southeastern
region of Brazil to define the southwestern-southern
boundary of the São Francisco craton and to determine
the electrical properties of the mantle beneath this area.
The MT data were acquired along four main profiles,
roughly perpendicular to the tectonic grain of different
parts of the craton and its margins (Fig. 2).
On the API profile, 25 stations were recorded along
180 km in a WSW–ENE direction. The profile crosses
the cluster of diatremes in the central part of the Alto
Paranaı́ba igneous province (APIP), with its western
limit over the Paraná basin and the eastern limit on
the Neoproterozoic sedimentary cover of the São Francisco craton (Bologna et al., 2005). A commercial
single-station broadband MT system (Metronix GMS05)
was used at every site in a coordinate system with
one of the horizontal axis aligned with the magnetic
meridian (N20◦ W). The telluric field variations were
measured with 100 m dipoles, in a cross-configuration,
with non-polarizable cadmium-cadmium chloride electrodes, whereas the magnetic fields were measured with
induction coils for the two horizontal and one vertical
components. The period range of this equipment is of
0.0008–1024 s and recording time was typically of at
least 24 h. At 14 of the above stations, other commercial
remote-referenced long-period MT systems (Phoenix
LRMT) were operated in the period range of 20 to at least
13,653 s with recording time of at least 2 weeks under
the same geomagnetic coordinate system. The horizontal
telluric fields were acquired with lead–lead chloride elec-
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A.L. Padilha et al. / Physics of the Earth and Planetary Interiors 158 (2006) 190–209
Fig. 2. Schematic outline of the main geological provinces in the study area with the identification of the four MT profiles. BB stands for the
Brazilian belt, RB for the Ribeira belt, AB for the Araçuaı́ belt, and AO for the Atlantic Ocean. Grey traces are the location and direction of main
structural elements (faults and shear zones). Locations of main cities are also indicated: São Paulo (SP), Rio de Janeiro (RJ), and Belo Horizonte
(BH).
trodes in a cross-configuration with 150-m lengths, while
the three components of the magnetic field were acquired
with high-sensitivity ring-core fluxgate sensors. At sites
where both systems were used, the data was merged to
obtain more than seven-decade period range.
The 230-km-long PIU profile, in an ENE–WSW
direction, started on the Brası́lia belt, in the western side,
and ended on the exposed Archean basement of the São
Francisco craton. The main profile comprised 16 singlestation broadband and 12 remote-referenced long-period
MT stations. A secondary branch, with four broadband
and long-period measurements, extended the survey
from near the centre of the main profile in the northwestern direction, crossing a kimberlitic province within the
Brası́lia belt. The long-period data were recorded with
the same systems used in the API profile, but the broadband data were measured with a new generation MT
system (Metronix GMS06). Recording times and useful period ranges were the same as in the API profile.
For both broadband and long-period systems, the telluric
field variations were measured with lead–lead chloride
electrodes with 150 m dipoles.
The same systems and field procedures were used
in the 160-km-long SJR profile in a roughly NS direction. The profile extended from Neoproterozoic paragneissic rocks in the Ribeira belt, at the southern end,
across a Mesoproterozoic pegmatitic province of the São
Francisco craton, and ended on the exposed Archean
basement of the craton. Broadband MT data were
acquired at 19 sites from which 11 also had long-period
data.
For the ∼550-km-long IBI profile, a new set of instruments was available, including another GMS06 and a
time domain electromagnetic (TDEM) system (Zonge
GDP-32II ). At most stations, time series in the broadband range were recorded simultaneously at pairs of
sites in order to use remote referencing for noise reduction in the analysis (Gamble et al., 1979) and TDEM
data were acquired for static shift correction of the MT
soundings (Meju, 1996). The EW profile comprised 27
broadband and 22 long-period stations and ran from the
Phanerozoic sediments and volcanics of the Paraná basin
to the west, across a regional fold with WNW plunging in the Neoproterozoic Brasilia belt in the centre,
and the Neoproterozoic/Phanerozoic cover and exposed
Archean complex of the São Francisco craton to the east.
After rotating the time series to geographic coordinates, modern techniques of robust processing were
applied to the data to yield MT impedance tensor and
vertical field geomagnetic transfer function estimates as
a function of period for each site. Data from the GMS05
system were previously edited to remove noisy segments and robust processing was used to calculate single
site estimates of the electromagnetic transfer functions
through the commercial software PROCMT (Metronix,
Germany). For the GMS06 and the long-period systems,
the time series data were processed using remote reference data when available (Egbert, 1997).
A.L. Padilha et al. / Physics of the Earth and Planetary Interiors 158 (2006) 190–209
The long-period and broadband MT data were then
merged into single responses for each station. Due to
differences in electrode array layout, the apparent resistivity curves of the long-period MT data were shifted
to the level of the broadband data at some sites of the
API profile. However, the absolute levels of the apparent resistivity curves at each site (static shifts) were
estimated as part of a two-dimensional (2D) inversion
procedure (Bologna et al., 2005).
Due to lack of significant cultural EM noise at most of
the sites and the high signal levels, high quality response
function estimates were obtained at most sites for the
whole period range, with errors in the impedance phases
of less than 2◦ . The only exceptions were observed at
some sites in the centre of the API profile, at sites near the
large Furnas Dam in the western side of the PIU profile,
and at the two northernmost sites of the SJR profile, the
closest to the large Belo Horizonte city.
Typical examples of high-quality data are shown in
Fig. 3, including one site from each profile. Site API78
was located close to the border of the Paraná basin. Phase
and apparent resistivity curves in orthogonal directions
showed a small split in the 0.01–100 s period interval,
indicative of a departure from the one-dimensional (1D)
195
condition in the crust. IBI121 was positioned at the western end of the IBI profile, over a thicker sedimentaryvolcanic package of the Paraná basin. Sites in this region
presented a phase split at periods between 100 and 1000 s
(not seen in the unrotated data of Fig. 3, but shown in
Fig. 4) related to structural or intrinsic anisotropy at the
base of the crust. PIU06 was located near the centre of
the PIU profile, over a thin Phanerozoic cover of the São
Francisco province and close to one of the teleseismic
stations. A significant split was observed in the phase and
apparent resistivity at short periods, indicative of multidimensional structure at shallow depths. SJR03 was
situated in the southern part of the SJR profile, over
the metamorphic rocks of the Ribeira belt. The shortest periods showed apparent resistivity split and divergent phases (lower than 45◦ ), probably associated with
inductive distortions in the electric field caused by the
contrasting heterogeneous topsoil cover of the resistive
Precambrian rocks.
The most striking result in these graphs is the
behaviour of the phase data for periods longer than 100 s
that are very similar in orthogonal directions. Excepting the sites in the western end of the IBI profile, this
result was confirmed for rotated data of all MT soundings
Fig. 3. MT responses derived from data acquired at sites representative of each profile (three first letters as the site identification). Apparent
resistivities and phases are from electric dipole oriented at geographic NS (triangles) and EW (squares) coordinates.
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A.L. Padilha et al. / Physics of the Earth and Planetary Interiors 158 (2006) 190–209
Fig. 4. Upper panels are the G–B parameters for unconstrained 2D fit to long-period data of site IBI121: (a) normalized chi-square misfit; (b) strike,
current channeling, shear and twist azimuths; (c) estimated scaled regional apparent resistivities (triangles in the strike direction and squares in the
perpendicular direction); (d) estimated regional phases. Lower panels (e–h) are the fit of the model data to the observed data for all four impedance
elements (squares and solid lines are the real parts, triangles and dashed lines are the imaginary parts).
A.L. Padilha et al. / Physics of the Earth and Planetary Interiors 158 (2006) 190–209
and indicated that the occurrence of electrical anisotropy
is not very pronounced at upper mantle depths for this
region of the Brazilian shield.
4. Geoelectric strike azimuths
The MT impedance tensor can be used to determine the geoelectric strike direction, a useful parameter
for defining the different geoelectric structures detected
by the data. In the maximum phase split orientation
method, the impedances are rotated to determine the
greatest phase difference between the off-diagonal elements. The geoelectric strike is then determined as
the orientation at which the phase difference between
off-diagonal impedance tensor elements is maximized.
However, electric charge accumulation on near-surface
heterogeneities can distort the measured MT response
so that the azimuth determined in this way may no
longer accurately represent the regional conductivity
structure (Jones and Groom, 1993). In the Groom–Bailey
(G–B) tensor decomposition method (Groom and Bailey,
1989), the geoelectric strike is determined simultaneously with the near-surface distortion effects using a
least-squares approach. The distortion model assumed is
of a galvanic 3D distortion sheet overlying a regional 2D
Earth. Because of the number of parameters fitted in this
method, stable estimates of the regional strike require its
estimation over relatively broad period bands.
The geoelectric strikes in the study area were derived
using the G–B tensor decomposition at nearly onedecade-wide bands (six or seven adjacent periods) in
the intervals of 4–27 s, 40–320 s, 430–2560 s, and periods longer than 3400 s. The tensor decomposition code
of McNeice and Jones (2001) was used in the singlesite, multi-frequency option on the MT responses from
each of the sites, with an assumed error floor of 2◦ in
phase. Fig. 4 shows the G–B model parameters and fit for
an unconstrained best-fitting 2D parameterization of the
long-period data of site IBI121. The upper panels show
the statistical residual (χ2 ) for the model, the G–B distortion parameters, and the 2D regional apparent resistivity
and phase estimates in the resulting strike-coordinate
frame. The details of the misfit are given in the lower panels, which show a scaled fit of the estimated impedance
tensor under the G–B model compared with the data.
In Fig. 4, the χ2 variable presents a value less than 4
(horizontal line in Fig. 4a) for all periods, indicative of
an acceptable model of distortion for reliable error estimates (Groom et al., 1993). The twist parameter is almost
period independent with values within ±5◦ , whereas the
shear is slightly larger at long periods but within the ±15◦
interval. The strike angle is around 20◦ (or −70◦ due to
197
the 90◦ ambiguity) at shorter periods and decreases to
a long-period azimuth of about 0◦ (or 90◦ ). Estimated
regional apparent resistivities and phases vary smoothly
over the whole period range, with scaterers and larger
error bars at periods longer than 10,000 s due to low
signal/noise ratio. The previously referred phase split at
periods between 100 and 1000 s is now clearly seen in
the rotated data. All off-diagonal and diagonal tensor
elements are well fit by the distortion model.
Similar analyses were performed at every site. Generally a good fit was observed to each element of the measured impedance tensor at most of the sites. Chi-squared
misfits were very small and shear and twist angles were
less than 15◦ , an indication that the sites were relatively
undistorted and that the geoelectric structure in the survey region fitted well with the 3D/2D approximation of
the G–B decomposition. Exceptions were observed at
the noisy sites previously described and at some localized sites in different profiles. At these sites, twist and
shear angles were very high, sometimes approaching the
limits of 60◦ and 45◦ , respectively. An additional analysis of the dimensionality of the impedance tensors was
made using the Bahr’s classification (Bahr, 1991). The
results indicated that most of the data fit a 2D model
distorted by local heterogeneities with only a weak to
strong galvanic response (classes 3–5), consistent with
the G–B decomposition.
The strike azimuths from single site decompositions
are shown in Fig. 5 for the four bandwidths previously
defined, with the lengths of the bars scaled by the average
phase difference over the band between the conductive
(strike) and resistive directions. For the shortest period
band, the azimuths can be compared to the geological
and tectonic features present at the surface as displayed
in Fig. 2, which indicate that the main structural elements trend predominantly NW and NNW directions
along the western border of the São Francisco craton
and E–W to ENE on the southern border of the craton.
Geoelectric strike directions in this band are, in general,
consistent with these predominant surficial geological
trends of the region. The map shows stronger site-tosite variation, associated with the complex upper crustal
structure that includes transport of large allochtonous terrains during the Neoproterozoic remobilization. Largest
lateral variations are observed in the API profile, with the
general NW direction prevailing in the southwestern end
of the profile, oscillating from WNW to E–W beneath
sites in the central area, and NNE off to the northeastern region. This anomalous feature is interpreted to be
related to the extensive magma emplacement during the
Cretaceous period (Bologna et al., 2005). For the next
three bands, strikes were defined through the direction
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Fig. 5. The unconstrained G–B regional strike from four period bands: (a) 4–27 s; (b) 40–320 s; (c) 430–2560 s; (d) >3400 s. The azimuths of the
solid bars illustrate the preferred geoelectric strike direction for that period band and the lengths of the bars are proportional to the average phase
difference between the off-diagonal elements of the recovered regional 2D impedance tensor.
of maximum phase. The maps show good site-to-site
consistency, with a remarkable coherence in the strike
directions from the different profiles. However, a large
number of sites exhibit small phase differences in orthogonal directions, implying a weak lateral variation of the
electrical resistivity beneath the area. This behaviour is
more evident at the sites on the western side of the São
Francisco craton, along IBI and PIU profiles. One significant rotation of the strike directions is observed at the
southernmost sites of the SJR profile, where azimuths
change from nearly E–W in the three first bands to NW
at the longest period band.
Other ways of assessing geoelectric strike and dimensionality of the region are provided by induction vectors.
They are determined independently only from the magnetic field variation recorded at each MT sounding. Over
a 2D structure, the real part of the induction vectors are
orthogonal to the geoelectric strike and can be used to
resolve the 90◦ ambiguity in the strike directions derived
from tensor decomposition. Over more complex structure, induction arrows can be influenced by regional
conductivity anomalies outside the MT study region generating a lack of orthogonality between strike and arrows
orientation (Simpson and Bahr, 2005). Fig. 6 shows maps
of the real induction vectors at periods representative of
the four bands for which geoelectric strikes were calculated. The vectors were derived from magnetic variations
measured with fluxgate magnetometers (long period MT
systems). The maps for the two longest periods also
include data from a regional magnetometer array study
(Subba Rao et al., 2003), also using fluxgate magnetometers (Chamalaun and Walker, 1982).
At the two shortest periods the vectors are almost
identical, indicating significant current concentration in
the region of the Alto Paranaı́ba volcanics on the API
profile and pointing towards larger conductance to the
centre of the Paraná basin, on the western side of the
IBI profile. The other soundings over the Brası́lia belt
and the São Francisco craton have negligible vertical
response at 20 s, and point towards a conductive structure
between the PIU and SJR profiles at 106 s. Vectors from
sites over the Ribeira belt points to the south, parallel to
the SJR profile. At the two longest periods, vectors are
near zero all over the northwestern side of the study area
and point southeast to the central and southeastern area,
towards the electric currents concentrated at the oceanland boundary. The significance of these results will be
discussed in more details in a later section that will
include a 3D forward modelling of a sketchy regional
conductance map.
A.L. Padilha et al. / Physics of the Earth and Planetary Interiors 158 (2006) 190–209
199
Fig. 6. Real induction vectors calculated from long-period MT (black bars) along the profiles and GDS (grey bars) from a wider survey. Periods
are: (a) 20 s; (b) 106 s; (c) 1280 s; (d) 6826 s (for MT data) and 7680 s (for GDS data). Vectors are reversed to point towards current concentrations.
5. Depth estimation
MT short periods sample near-surface structures
whereas long periods are sensitive to deeper structures.
Therefore, the period dependence of the geoelectric
strike can be used to distinguish between lithospheric
and sublithospheric effects, helping to resolve the ambiguity associated with the depth of seismic anisotropy.
However, due to the diffusive nature of EM propagation and the vast range in electrical conductivity inside
the Earth, one must be careful that the MT and seismic
information derives from the same depths. Also, there are
Earth structures for which EM signal penetrations have
radically different depths for the two orthogonal modes
of propagation (as for instance, site PIU06 of Fig. 3).
Because of this, comparisons of the MT responses
observed at the same period range from different locations with seismic anisotropy derived from shear-wave
splitting analysis are not straightforward (see also Jones,
2006).
MT depth estimations can be hindered by near-surface
distortions of the electric field amplitudes, causing a
static shift in the impedance magnitudes. For sites where
TDEM data were unavailable, the static shifts were estimated as part of the 2D inversion procedure for each
profile (Bologna et al., 2005; Pádua, 2004). The approach
used was to fit well the phase data at all stations and the
apparent resistivities from stations without static shift,
yet allowing a larger misfit to the apparent resistivity
data of static-shifted stations (Wu et al., 1993). The static
shift values estimated in this way were used to correct
each site prior to depth estimation.
However, one cannot assume that the electric field
is distorted only by these near-surface heterogeneities
because conductive structures present in the middle and
lower crust can affect the electric field at longer periods in similar way (Bahr et al., 2000). As the galvanic
distortions of these structures at different depths are summarized at long periods, they can be interpreted as a
single distorter. The total static shift effect can then be
corrected using a global sounding curve (Vanyan et al.,
1980) or magnetic transfer functions derived from Sq
variations (Bahr et al., 1993). These techniques for correction of the distorted apparent resistivity data are of
limited applicability in complex 3D medium inhomogeneities and when array data are not available.
To avoid inaccurate correction for these galvanic distortions we opted for a conservative approach, excluding
from the analysis, at a given period band, all geoelectric
strikes affected by conductive bodies at shorter periods.
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Also, because G–B decomposition gives stable results
only if the phase split is much greater than the errors of
phase measurements, we excluded from the analysis the
strikes for which the 2D regional impedances do not have
significantly different phases. Following Berdichevsky
(1999) and considering the mean deviation of 2◦ in the
impedance phases, we chose 7◦ as the minimum phase
split for a valid G–B decomposition.
The penetration depths of the MT signals in the four
period bands were estimated using skin depth relations
and the method of Schmucker and Jankowski (1972).
Calculations were checked with an approximate equivalent depth using the formulation of Niblett and SaynWittgenstein (1960).
Periods of 4–27 s correspond to penetration to the
base of the crust, excepting at the westernmost sites of the
IBI profile where larger conductance of the Paraná basin
limit penetration to the middle crust. MT strikes in this
band are therefore representative of the middle and lower
crust. Periods of 40–320 s typically penetrated to depths
between 50 and 100 km into the upper mantle. Again,
penetration is smaller at some sites to the centre of the
Paraná basin. For the 430–2560 s band, most sites in the
Ribeira and Brası́lia belts and some isolated sites close to
the border of the Paraná basin have EM fields penetrating
to the base of the lithosphere (depths of 150–200 km).
Surprisingly, most sites over the exposed Archean rocks
of the São Francisco craton do not have penetration to
such depths at these periods. This is associated with
an anomalously large conductivity of the upper mantle beneath this region, probably affected by Mesozoic
tectono-thermal reactivation (Pádua, 2004). For periods
longer than 3400 s, some sites in the Ribeira and Brası́lia
belts, two sites close to the Paraná basin border and only
one site over the Archean São Francisco craton have
penetration deeper than 250 km, providing information
probably about sublithospheric depths.
5.1. Crustal strike
Fig. 7 presents a compilation of seismic anisotropy
derived from measurements of SKS and SKKS splitting in Southeast and Central Brazil. Description of
data acquisition, processing and details of interpretation were given by James and Assumpção (1996) and
Heintz et al. (2003). The figure also shows the direction of present-day absolute plate motion (APM) for
the HS3-NUVEL1A model (Gripp and Gordon, 2002),
which is determined assuming that hotspots are stationary relative to the deep mantle below the asthenosphere.
There is a significant discrepancy between the seismic
anisotropy and the APM direction, with the fast polarization direction of the seismic waves showing preferred
orientation generally parallel to the last major orogenies. In the Brası́lia belt, this direction has a NW trend in
agreement with a proposed collision between a cratonic
Fig. 7. Azimuths and amplitudes of shear-wave splitting (open bars) and geoelectric anisotropy (solid bars), calculated for EM penetration depths
between 10 and 40 km. For electrical measurements, the solid bars indicate the most conductive directions and their lengths are proportional to the
phase difference between the most conductive and orthogonal directions. For teleseismic data, the open bars indicate the polarization direction of
the fast shear wave and their lengths are proportional to the delay time between the two split waves. Direction of present-day absolute plate motion
(APM) of the South American plate is also indicated.
A.L. Padilha et al. / Physics of the Earth and Planetary Interiors 158 (2006) 190–209
block beneath the Paraná basin and the São Francisco
craton, in the earliest stages of Gondwana amalgamation, by which the southern section of the Brasilia belt
was formed (Alkmim et al., 2001). In the southern part
of the Ribeira belt, the anisotropy appears to be stronger
(>1 s) with a WSW direction, parallel to the trend of conspicuous transcurrent shear zones. To the south of the
São Francisco craton, in the interference zone between
the Brası́lia and Ribeira belts, the anisotropy pattern is
more complex, with no predominating direction. In the
central part of the Paraná basin, where also there are no
electromagnetic data, the anisotropy direction tends to
turn to E–W.
Fig. 7 also presents the selected geoelectric strikes
for EM signals with typical penetration at crustal depths
(10–40 km). As previously explained, they were derived
from the period band between 4 and 27 s. At the southern
border of the São Francisco craton and along the Brası́lia
belt, geoelectric strike is consistent with the main directions of the seismic anisotropy and the regional structural
grain. The general NNW trend in the centre part of the
craton is in agreement with the directions of emplacement of mafic dykes in this region. There are no teleseismic data in the APIP volcanic region (API profile)
and the conspicuous lateral variation of the geoelectric
strike is related to differences in the Mesozoic intrusive
and extrusive magmatism affecting mid-crustal structures in different ways along that MT profile (Bologna
et al., 2005). No electric strike information is available
for most of the Ribeira belt except along the region near
the southern cratonic border.
201
A significant disagreement between seismic
anisotropy and geoelectric strike directions is observed
at sites over the northeastern Paraná basin. In this
case, the geoelectric data follow the known structural
trend beneath the basin, given by gravity anomalies.
The position and direction of the largest geoelectric
strike bars are consistent with a steep gravity gradient
interpreted as an ancient suture zone (Lesquer et al.,
1981). Yet, towards the central part of the Paraná basin,
no electrical azimuth is available.
5.2. Topmost upper mantle strike
Fig. 8 presents geoelectric strikes derived from EM
signals with penetration at depths between 50 and
100 km. Results in the southern part of the São Francisco
craton are similar to the ones from the crust, rotating
approximately from WSW in the limits of the Ribeira
belt to E–W or WNW inside the São Francisco province.
At sites on the exposed Archean rocks of the craton, the
geoelectric strikes point NW, in accordance with the seismic data from this region. In spite of the rigorous data
selection, some stations over the Passos nappe (PIU profile) still appear to be affected by noise and the general
NW trend is not easily seen in this region.
A remarkable alignment in the WNW direction is seen
in the electrical azimuths of the IBI stations over the
Brası́lia belt. This direction is the same as for the fault
zones in the area. The gravity gradient is still sensed at
these depths, indicating that the suture zone extends deep
into the upper mantle. To the west of the gradient, the
Fig. 8. The same as in Fig. 7, for geoelectric anisotropy at depths between 50 and 100 km.
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Fig. 9. The same as in Fig. 7, for geoelectric anisotropy at depths between 150 and 200 km.
vectors rotate smoothly to the NW direction, tending to
align with the seismic anisotropy direction.
5.3. Lowermost lithosphere strike
Fig. 9 shows the geoelectric strikes at depths in the
interval of 150–200 km. There is an excellent correlation
with seismic anisotropy at every region where both data
sets are available. The regional NW main direction is
only disturbed at the southern part of the São Francisco
craton, where the E–W trend observed at lower depths
still persists, and at the northeastern end of the API profile, where rift-related Mata da Corda volcanics outcrops
and the direction of the geoelectric strike seems slightly
rotated to NNW.
5.4. Sublithospheric strike
Fig. 10 presents geoelectric strikes at the sites with
EM signals reaching depths greater than 250 km, presumably in the sublithospheric mantle. Generally, the
directions are still in agreement with the seismic data
Fig. 10. The same as in Fig. 7, for geoelectric anisotropy at depths greater than 250 km.
A.L. Padilha et al. / Physics of the Earth and Planetary Interiors 158 (2006) 190–209
but the general NW trend is significantly different from
the absolute plate motion and is therefore inconsistent
with interpretations in terms of plate-scale mantle flow or
asthenospheric deformations. Local differences in comparison to the seismic anisotropy are observed in the
transition area where the southern São Francisco craton
abuts the western Ribeira belt, characterized by a clear
rotation in the direction of the geoelectric strike from
roughly E–W at lithospheric depths to NW at the deeper
sublithospheric mantle. Also, in the APIP region, the
NNW electric trend does not agree with the E–W seismic anisotropy observed at one station about 150 km to
the north of the API profile.
6. Numerical simulation of GDS and MT
responses
To associate unequivocally the observed MT strike
directions with the presence of anisotropy, it is necessary
to disqualify possible effects related to regional-scale
heterogeneities. A 3D forward modelling was used to
test different conductivity structures in an attempt to fit
qualitatively the available GDS transfer functions and
some characteristics of the MT responses. The surface
response of the 3D models was calculated using the code
of Mackie et al. (1994).
The model was built with a first layer representing
the upper crust, from the surface to a depth of 5 km,
including ocean bottom sediments and seawater. The
continental shield consists of a highly resistive crust,
with about 30 S of total conductance. It was not necessary to include continental sediments in the model
because they have minor influence on long period data.
This surficial layer was embedded in a layered 1D model,
derived as an approximate average from the models of
this portion of the Brazilian shield (Bologna et al., 2005,
2006; Pádua, 2004). The resistive continental shield is
bordered by the highly conductive Atlantic Ocean to the
east and southeast (conductance of 10,000 S, following
Padilha et al., 2002). To test the anisotropy hypothesis, an anisotropic layer was included at upper mantle
depths, represented by alternating conductive and resistive dyke-like medium. The ratio of integrated lateral
average conductivities (across and along strike) reproduces the value of the anisotropic layer and the strike
angle of the best conductor is around N65◦ W, in accordance with the seismic anisotropy. To simulate the heterogeneity hypothesis, 2D regional-scale structures were
included at the continental lower crust. A trial-and-error
approach was used in an attempt to fit the available data.
The results of the numerical calculations for the
anisotropy hypothesis are summarized in Fig. 11. In
203
spite of the quite simple 3D model chosen to represent
the regional structure there is a remarkable agreement
between synthetic and experimental data. The induction
arrows at most of the sites are clearly controlled by the
behaviour of the coast effect over most of the study area.
The GDS response is large close to the boundary between
the seawater and the land mass and decrease away from
that boundary, vanishing at the distant sites. The figure also shows geoelectric strikes determined from the
impedance tensor response at a selected site in the surface of the model. Consistent with the 1D layered model,
the derived strike directions tend to align around −65◦
(N65◦ W) at periods sensing the anisotropic layer. This
direction is not normal to the observed and calculated
induction arrows at this site (difference of 20◦ between
the azimuths of the arrows and the geoelectrical strike).
These results indicate that the ocean accounts for the
tipper magnitude and direction, whereas the anisotropic
layer in the upper mantle is responsible for the MT strike.
On the other hand, simulations with the crustal heterogeneity model were unable to reproduce satisfactorily
the GDS and MT results. In fact, the coast effect that controls the magnetic transfer functions is not overprinted
by any significant inland regional conductivity anomaly.
It must be observed that localized crustal conductors are
undetected at the regional scale of these data (spacing
around 100 km between stations), but small structures
are unlikely to generate the regional phase split pattern
observed at long periods (see Fig. 5).
As the influence of large-scale lower crustal conductors appears to be negligible in this region of the Brazilian
shield, upper mantle anisotropy is favoured to explain
strikes and phase split in the long period data. This
result is at variance to that observed, for instance, in the
Fennoscandian shield (Korja, 2003). That shield hosts
high conducting elongated belts, possibly signatures of
previous tectonic processes, concentrating induced currents that can generate the great majority of the strikes
and apparent anisotropy observed around them. Similar structures are not observed in our study area where
conductive lithospheric bodies appear as isolated pockets, most of them related to the voluminous magmatism
that occurred from Early Cretaceous until Eocene times
(Bologna et al., 2005, 2006; Pádua, 2004). Such characteristic is independently confirmed by regional seismic
tomography and geoid anomaly studies (Schimmel et al.,
2003; Molina and Ussami, 1999).
7. Discussions
In general, the spatial behaviour of phase splitting
and amplitude of induction vectors help to distinguish
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Fig. 11. Summary of 3D modelling to test the anisotropy hypothesis. (a) Map of conductance of the surface layer (0–5 km); (b) lateral plan view
of the 1D layered model incorporating the surficial layer (thin sheet) and an anisotropic layer from 100 to 160 km depths; anisotropy strike of
N65◦ W and resistivities of blocks within the anisotropic layer alternating with 300 and 30 m; (c) comparison between observed (black arrows)
and calculated (grey arrows) real GDS induction arrows for a period of 1280 s; (d) strikes from synthetic MT data at the site circled in (c), calculated
according to Swift (1967) and Bahr (1991), shown, respectively, as grey and black squares.
between intrinsic anisotropy and the presence of lateral structure as the cause of geoelectric strike. In the
laterally displaced conductivity case, the phase splitting will decrease with increasing site distance from the
anomaly, whereas in deep anisotropy the splitting will
remain the same over large distances. On the other hand,
magnetic fields are generated by lateral conductivity gradients. Therefore, the absence of a significant induction
vector or phase splitting not correlated with the geomagnetic transfer functions supports the hypothesis of deep
anisotropy.
MT data imaging crustal depths along the API profile
and those crossing a gravity-defined suture zone beneath
the Paraná basin have significant lateral variation in the
geoelectric strike and expressive induction vectors at
short periods. Both are clear examples of lateral structure controlling the geoelectric strike. On the other hand,
the general NW geoelectric strike that pervades from the
crust down to sublithospheric depths and is not associated with locally generated vertical magnetic field at long
periods are here interpreted as being related to regional
geoelectric anisotropy. Small phase splitting at periods
longer than 100 s at most of the MT soundings suggests
a weak electric anisotropy in the upper mantle beneath
most of the region.
At sites where phase splitting is small even within the
crust, it is possible to use 1D inversion to derive the range
of anisotropic conductance that can fit the data. An example of 1D layered-earth inversion is shown in Fig. 12 for a
site in the southern part of the São Francisco craton, using
data rotated to the upper mantle strike direction. The
approximate depth transformation (Niblett and SaynWittgenstein, 1960) is also shown and indicates that penetration depths of TE and TM data are virtually the same
A.L. Padilha et al. / Physics of the Earth and Planetary Interiors 158 (2006) 190–209
205
Fig. 12. Results of 1D layered-earth inversion of rotated apparent resistivity and phase data for site SJR03 of Fig. 3. Left panel shows comparisons
between the observed and calculated along-strike (black) and across-strike (grey) responses. Right panel shows the linearized inverse layeredearth models that fit the data (grey zone is the anisotropic zone) and approximate depth transformation of the experimental data (Niblett and
Sayn-Wittgenstein, 1960).
at depths around 100 km. An anisotropic layer is required
at depths greater than 120 km, with a conductance ratio
of about 3. This is a typical result, with anisotropy factors not exceeding 3 for most of the sites. Such small
anisotropy is readily explained by olivine LPO in the
upper mantle, with conductivity locally enhanced by the
diffusion of hydrogen resulting from addition of small
amounts of water in the olivine crystal lattice at regions
subjected to mantle metasomatism. It must be noted
that this hypothesis requires that some remnant hydrogen would be retained in the upper mantle for the long
period since the last tectono-thermal event liable for the
metasomatism. Other alternative sources of enhanced
conductivity in the upper mantle include a free aqueous
fluid phase, conductive minerals (graphite, sulphides)
and partial melts (Jones, 1999). However, they are less
likely to explain the observed conductivity distribution
in this region of the Brazilian shield (see discussion in
Bologna et al., 2006). These qualitative observations still
need to be confirmed by quantitative modelling, linking
electric and seismic anisotropy to anisotropic hydrogen
diffusion in olivine, considering the characteristics of the
study area (e.g., Simpson, 2002).
The observed close resemblance between both results
is indicative of coincidental parallel seismic and geoelectric anisotropies in this region of the Brazilian shield,
similar to what is seen in Australia and Canada (Simpson,
2001; Eaton et al., 2004). The spatial coherence between
both anisotropies and surface structural features at the
western boundary of the São Francisco craton supports also a model of coherent lithospheric deformation derived from the last significant tectonic episode
rather than related to the mechanisms of absolute plate
motion. Similar results observed elsewhere have been
interpreted as evidence that both crust and upper mantle have remained undeformed since the last lithospheric
deformation episode, despite plate motion (Silver, 1996).
Simple asthenospheric flow is therefore rejected as
the source of mantle anisotropy around this region of the
Brazilian shield. Absolute motion of the South American plate has a well-defined trend, approximately E–W
for most of the continent (see Figs. 7–10). Observed fast
seismic azimuths and geoelectric strikes around the São
Francisco craton show no correlation with this direction
but display a striking correlation with the surface geology. Towards the centre of the Paraná basin, SKS fast
polarization directions are nearly parallel to APM. However, towards the western portion of the basin and over
the Rio Apa block (not shown in our figures) the seismic anisotropy is again aligned with the N–S direction
of crustal blocks bordering the western side of the basin
(James and Assumpção, 1996). It could be argued that
the asthenospheric flow would be channelled around the
topography at the base of the lithosphere, as suggested to
occur in central Europe (Bormann et al., 1996). However,
modelling of asthenospheric flow at the southern termi-
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nation of the craton failed to explain the sharp curvature
of the anisotropy pattern in the Ribeira and Brası́lia belts
(Heintz et al., 2003).
The geoelectric information can be used to solve the
problem of lack of vertical resolution of the seismic data.
Our results suggest that the NW anisotropy around the
southern and southwestern border of the São Francisco
craton extends beneath the continental plate, possibly at
least to the 410 km olivine–spinel transition. Very deep
lithosphere or coupling between lithospheric and sublithospheric mantle since the last major tectono-thermal
event is implied from the apparent absence of sublithospheric lateral mantle flow or deformation due to the
present-day westward motion of the South American
plate.
This interpretation is consistent with an independent
evidence for a mechanically coherent translation of the
South American plate and its upper mantle over the
last 130 Ma to explain an upper mantle low velocity
anomaly beneath Brazil revealed by seismic tomography
(VanDecar et al., 1995; Schimmel et al., 2003). Also,
Heintz et al. (2003) in their study at the Ribeira belt
found two stations with very high delay times requiring either an intrinsic anisotropy of mantle rocks much
larger than commonly described in petrophysical studies
or an anisotropic layer thicker than the lithosphere. On
the other hand, it must be considered that temperatures
in the 250–410 km depth range exceed 1000 ◦ C, and are
probably too high to allow sustained frozen anisotropy
in a mechanically coherent lithosphere/asthenosphere lid
on geologically relevant time scales (Vinnik et al., 1992).
A possible explanation for the anisotropy to continue
down to these depths could be a variation in grain size
with depth, allowing to an alignment of mineral grains
through dislocation creep (Ji et al., 1994). Furthermore,
the notion that the upper mantle has remained mechanically unaltered since the last tectono-thermal event down
to the transition zone places strong constraints on proposals of plate motion driven by a plate basal drag.
For example, Russo and Silver (1996) proposed for the
South American plate a mechanism to move this nonsubducting plate through basal tractions arising from
deep convective flow of the mantle because the stresses
required to form and maintain the Andes chain are too
high to be accounted only by ridge-push.
Another point to consider is the role of the Cretaceous intraplate magmatism. The NNW trend of the
deep geoelectric strike in the northernmost MT profile
is suggested to be associated with rift-related volcanics
(Bologna et al., 2006). Recent studies have shown that
rifting does not necessarily erase the inherited seismic anisotropy of a previously deformed lithosphere
(Vauchez and Garrido, 2001). Instead of that, rift orientation seems to be controlled by the preexisting lithospheric mantle fabric revealed by deep geophysical data
(Tommasi and Vauchez, 2001). Geoelectric anisotropy
related to this rift structure could be then related to preexisting mantle fabric, possibly reactivated by the nearby
collisional orogen but not necessarily correlated with its
stress perturbations to the lithosphere because of the significant time difference between both events.
Deep structures beneath the southern part of the
São Francisco craton and surrounding fold belts are
extremely complex. As a consequence, information coming from different geophysical methods is sometimes
difficult to reconcile. For example, surface and body
wave tomography have shown that this part of the craton is characterized by high velocities down to at least
200 km, indicating the presence of a thick and undeformed lithosphere beneath this region (Schimmel et
al., 2003). On the other hand, a possible explanation
for a circular (600–800 km in diameter), positive, 8 mamplitude geoid anomaly, detected in the same region
(Molina and Ussami, 1999) where MT data along profiles
PIU and SJR points to a circular-shaped high conductivity anomaly at upper mantle depths beneath the Archean
rocks of the craton (Pádua, 2004), requires a regional
thermal anomaly of a few tens of degrees at upper mantle depths to compensate for a modelled thinner crust.
Such results imply that the base of the lithosphere could
have been eroded through reheating.
Two-dimensional inversions of the MT data along the
four profiles have shown that the effects of the Neoproterozoic Brasiliano orogeny seems to be restricted
to the brittle and intermediate crust, while the underlying lithosphere remained unaffected. Moreover, the
borders of the blocks involved in the continental collision that generated the marginal belts are not detected
by the geophysical studies. Excepting anomalous areas
clearly affected by Mesozoic thermal events, the lithosphere beneath these belts presents high resistivities up
to 200 km (Pádua, 2004; Bologna et al., 2006). These
results are at variance with tomographic seismic models
that suggest a lithosphere thickness of around 100 km
beneath the belts surrounding the craton (Schimmel et
al., 2003).
Rotation of geoelectric strikes at depths sensing lithospheric and sublithospheric mantle over the Ribeira belt
brings another piece to these puzzling results. The sense
of rotation, with an E–W to WSW azimuth at the deep
lithosphere and a NW azimuth below it, is exactly the
opposite of what should be expected in the most commonly used models of two-layer anisotropy. These models would consider a lower layer corresponding to the
A.L. Padilha et al. / Physics of the Earth and Planetary Interiors 158 (2006) 190–209
APM direction in the sublithospheric mantle (N253◦ E)
and an upper layer related to the orogenic direction of the
Brası́lia belt (roughly NW). However, numerical simulations carried out with this model could not reproduce
the observed seismic variations at stations of this region
(Heintz et al., 2003).
Complexity of this region is certainly related to the
collision pattern of the Brasiliano orogens. They include
a roughly NW zone of collision of the São Francisco and
Paraná blocks, in the period between 630 and 600 Ma,
and the roughly ENE zone of collision of a near-coast
microplate with the southeastern border of the São Francisco paleocontinent, near simultaneously between 605
and 550 Ma (Heilbron et al., 2000). Frozen anisotropy is
related to the last tectonic event but in this case it must be
considered that minerals cannot reorient instantaneously,
so that the preferred orientation is a complicated function of the strain history, depending on how the response
varies with time and with the amount of strain, in addition to other factors such as temperature, strain rate, and
initial conditions (Wenk and Christie, 1991). Intricate
interactions between both collisions, in the zone where
they join in southern border of the craton, have generated a complicated type of deformation that is difficult
to solve with the available data.
8. Summary and conclusions
Complementary anisotropy measurements of seismic
and magnetotelluric (MT) techniques carried out separately in central-southeastern Brazil help to constrain
the manner anisotropy varies with depth, a key information concerning mantle motions and ancient continental fabrics. Forward modelling of a simplified regional
conductance map supports predictive interpretation of
deep electrical anisotropy in this region. MT responses
at long periods indicate that the conductive structure
is only weakly anisotropic at upper mantle depths so
that alignment of olivine with the influence of additional mechanisms at localized regions (e.g., diffusion of
hydrogen) is a ready explanation of most of the observed
results. The southwestern border of the São Francisco
craton presents resistive lower crust and upper mantle, thus allowing the deep penetration of EM signals.
Electric strikes derived at different depths along this
region match the fast polarization direction of S-waves
and show that lithospheric and sublithospheric deformation is vertically coherent with the surficial tectonic
grain, in a general NW direction. There is a significant
discrepancy between this direction and that related to
the present-day absolute plate motion, an indication that
mantle deformation is not dominated by strains associ-
207
ated with the basal drag linked to the westward motion
of the South American plate. The presence of a fossil
anisotropy since the last tectono-thermal event is interpreted as related to a very deep lithosphere or a coupling between lithospheric and sublithospheric mantle
beneath this region. Thus, the strain-induced crystallographic fabric formed in response to large-scale tectonic
processes in the ancient past probably extends to very
large depths. The long term persistence of this deep fossil
anisotropy has serious implications to mantle rheology
and the dynamics of plate motions.
Different electric azimuths from the general NW
trend are observed in the northern and southern part of
the study area. In the north, the orientation of electric
strikes seems to be aligned with rift-related magmatism, whereas in the south, a complex deep structure
is observed, probably related to nearly simultaneous
oblique collisions. More detailed geophysical studies at
both areas and also covering a wider geographic region
will be necessary to substantiate the present conclusions.
Acknowledgements
This study was supported by research grants and
fellowships from FAPESP (95/0687-4, 00/00806-5,
01/02848-0 and 03/10817-2) and CNPq (142617/97-0,
350683/94-8 and 351398/94-5). Relevant logistical support from SOPEMI S.A. (De Beers Brasil Ltd.) is also
acknowledged. The authors are grateful to Sérgio Fontes,
for loaning the long-period MT systems of ON/MCT for
the first fieldworks, François Chamalaun, for loaning the
fluxgate magnetometers of Flinders University for the
GDS array, and the dedicated field and lab crew. The
original version was improved by careful reviews from
two anonymous reviewers. Special thanks to Alan Jones
and Dave Eaton, guest editors of this issue, for encouragement and useful suggestions.
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